1. Field of the Invention
The present invention relates to a semiconductor device composed BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride) or TlN (thallium nitride) or an III-V group nitride based semiconductor (hereinafter referred to as a nitride based semiconductor) which is their mixed crystal and a method of fabricating the same.
2. Description of the Background Art
In recent years, GaN based light emitting semiconductor devices have been put to practical use as light emitting semiconductor devices such as light emitting diodes or semiconductor laser devices which emit light in blue or violet. Transistors using GaN based semiconductors have been also proposed. In such GaN based semiconductor devices, electrodes in ohmic contact with the GaN based semiconductor are required.
FIGS. 6 and 7 are schematic cross-sectional views showing the steps of an example of a method of fabricating a conventional GaN based light emitting diode.
As shown in FIG. 6(a), a GaN buffer layer 22, an n-type GaN layer 23, and a p-type GaN layer 24 are first successively formed on a sapphire substrate 21. Thereafter, a partial region from the p-type GaN layer 24 to the n-type GaN layer 23 is etched by an RIE (Reactive Ion Etching) method or the like, to expose the n-type GaN layer 23.
As shown in FIG. 6(b), a photoresist pattern 31 is then formed on the p-type GaN layer 24 and on the exposed upper surface of the n-type GaN layer 23. The photoresist pattern 31 has an opening 32 on the p-type GaN layer 24. In this state, a Pt film 25a having a thickness of 2000 xc3x85 is formed by an electron beam evaporation method on the p-type GaN layer 24 inside the opening 32 and on the photoresist pattern 31.
As shown in FIG. 6(c), the Pt film 25a on the photoresist pattern 31, together with the photoresist pattern 31, is then removed by a lift-off method.
As shown in FIG. 7(d), a photoresist pattern 33 is then formed on the p-type GaN layer 24, on a p electrode 25, and on the exposed upper surface of the n-type GaN layer 23. The photoresist pattern 33 has an opening 34 on the exposed upper surface of the n-type GaN layer 23. In this state, a Ti film 26a having a thickness of 200 xc3x85 and an Al film 27a having a thickness of 5000 xc3x85 are successively formed by an electron beam evaporation method on the upper surface of the n-type GaN layer 23 inside the opening 34 and on the photoresist pattern 33.
Further, as shown in FIG. 7(e), the Ti film 26a and the Al film 27a on the photoresist pattern 33, together with the photoresist pattern 33, are removed by a lift-off method. Thereafter, heat treatment for five minutes is carried out at a temperature of 600xc2x0 C. in an N2 gas atmosphere.
In the above-mentioned manner, p-side and n-side ohmic electrodes are formed.
The p electrode 25 composed of a Pt film in the above-mentioned conventional GaN based light emitting diode has a weak adhesive force. Accordingly, the p electrode 25 is easily stripped in the subsequent processes such as wire bonding. Further, the p electrode 25 is stripped relatively simply when a measuring needle is brought into contact therewith in examination of ohmic characteristics, for example.
On the other hand, alloying by heat treatment is required in order to obtain ohmic contact. Further, the p electrode 25 and an n electrode 30 must be formed in different processes. Consequently, the processes of forming the p electrode 25 and the n electrode 30 become complicated, so that it takes long.
An ohmic electrode having a laminated structure of a Ti film and an Au film has been also proposed. The ohmic electrode can be formed without being alloyed by heat treatment. However, the resistance of the ohmic electrode is easily increased at the time of heat-treating the other layers.
An object of the present invention is to provide a semiconductor device comprising a stable ohmic electrode which is not easily stripped and can be formed in a simple process on a nitride based semiconductor.
Another object of the present invention is to provide a method of fabricating a semiconductor device in which an ohmic electrode which is not easily stripped and is stable can be formed in a simple process on a nitride based semiconductor.
A semiconductor device according to one aspect of the present invention comprises a p-type nitride based semiconductor; and an ohmic electrode formed on the p-type nitride based semiconductor. The ohmic electrode comprises a first metal film having a thickness of not less than 3 xc3x85 nor more than 150 xc3x85 which is formed in contact with the p-type nitride based semiconductor and is composed of titanium, and a second metal film which is formed in contact with the first metal film and is composed of platinum or palladium.
In the ohmic electrode in the semiconductor device, the first metal film composed of titanium has the function of cleaning the surface of the p-type nitride based semiconductor, to make ohmic contact between the second metal film composed of platinum or palladium and the p-type nitride based semiconductor easy. Consequently, the ohmic electrode is formed in a simple process without being alloyed by heat treatment.
Since the first metal film composed of titanium has a strong adhesive force to the p-type nitride based semiconductor, an adhesive force of the ohmic electrode to the p-type nitride based semiconductor is improved. Consequently, the ohmic electrode is not easily stripped.
Particularly, the thickness of the first metal film is preferably not less than 3 xc3x85 nor more than 130 xc3x85, more preferably not less than 3 xc3x85 nor more than 130 xc3x85, still more preferably not less than 3 xc3x85 nor more than 50 xc3x85, and most preferably not less than 3 xc3x85 nor more than 10 xc3x85. Consequently, sufficient ohmic characteristics can be obtained while preventing the ohmic electrode from being stripped.
The semiconductor device may further comprise a third metal film which is formed in contact with the second metal film and is composed of gold.
Thus, wire bonding can be easily performed. In this case, since the ohmic electrode composed of the first metal film and the second metal film can be formed without being alloyed by heat treatment, the first metal film, the second metal film and the third metal film can be formed in the same process.
The p-type nitride based semiconductor may contain at least one of boron, gallium, aluminum, indium and thallium.
A semiconductor device according to another aspect of the present invention comprises an n-type nitride based semiconductor; and an ohmic electrode formed on the n-type nitride based semiconductor. The ohmic electrode comprises a first metal film having a thickness of not less than 3 xc3x85 nor more than 100 xc3x85 which is formed in contact with the n-type nitride based semiconductor and is composed of titanium, and a second metal film which is formed in contact with the first metal film and is composed of platinum or palladium.
In the ohmic electrode in the semiconductor device, the first metal film composed of titanium has the function of cleaning the surface of the n-type nitride based semiconductor, to make ohmic contact between the second metal film composed of platinum or palladium and the n-type nitride based semiconductor easy. Consequently, the ohmic electrode is formed in a simple process without being alloyed by heat treatment.
Since the first metal film composed of titanium has a strong adhesive force to the n-type nitride based semiconductor, an adhesive force of the ohmic electrode to the n-type nitride based semiconductor is improved. Consequently, the ohmic electrode is not easily stripped.
Particularly, the thickness of the first metal film is preferably not less than 3 xc3x85 nor more than 50 xc3x85, more preferably not less than 3 xc3x85 nor more than 30 xc3x85, and still more preferably not less than 3 xc3x85 nor more than 10 xc3x85. Consequently, sufficient ohmic characteristics can be obtained while preventing the ohmic electrode from being stripped.
The semiconductor device may further comprise a third metal film which is formed in contact with the second metal film and is composed of gold.
Thus, wire bonding can be easily performed. In this case, since the ohmic electrode composed of the first metal film and the second metal film can be formed without being alloyed by heat treatment, the first metal film, the second metal film and the third metal film can be formed in the same process.
The n-type nitride based semiconductor may contain at least one of boron, gallium, aluminum, indium and thallium.
A semiconductor device according to still another aspect of the present invention comprises a p-type nitride based semiconductor; an n-type nitride based semiconductor provided so as to be brought into contact with the p-type nitride based semiconductor; a first ohmic electrode formed on the p-type nitride based semiconductor; and a second ohmic electrode formed on the n-type nitride based semiconductor. The first ohmic electrode comprises a first metal film having a thickness of not less than 3 xc3x85 nor more than 100 xc3x85 which is formed in contact with the p-type nitride based semiconductor and is composed of titanium, and a second metal film which is formed in contact with the first metal film and is composed of platinum or palladium. The second ohmic electrode comprises a third metal film having a thickness of not less than 3 xc3x85 nor more than 100 xc3x85 which is formed in contact with the n-type nitride based semiconductor and is composed of titanium, and a fourth metal film which is formed in contact with the third metal film and is composed of platinum or palladium.
In the first ohmic electrode and the second ohmic electrode in the semiconductor device, the first and third metal films composed of titanium respectively have the functions of cleaning the surfaces of the p-type nitride based semiconductor and the n-type nitride based semiconductor, to make ohmic contact between the second metal film composed of platinum or palladium and the p-type nitride based semiconductor and ohmic contact between the fourth metal film composed of platinum or palladium and the n-type nitride based semiconductor easy. Consequently, the ohmic electrode is formed in a simple process without being alloyed by heat treatment.
Since the first and third metal films composed of titanium respectively have strong adhesive forces to the p-type nitride based semiconductor and the n-type nitride based semiconductor, respective adhesive forces of the first ohmic electrode and the second ohmic electrode to the p-type nitride based semiconductor and the n-type nitride based semiconductor are improved. Consequently, the first and second ohmic electrodes are not easily stripped.
Furthermore, the first ohmic electrode and the second ohmic electrode have the same structure, and are composed of the same material. Accordingly, the first and second ohmic electrodes can be simultaneously formed in the same process. Consequently, the number of fabricating processes is reduced, thereby making it possible to shorten fabricating time.
As a result, ohmic electrodes which are stable and reliable can be easily formed, respectively, on the p-type nitride based semiconductor and the n-type nitride based semiconductor.
The thickness of the first and third metal films is preferably not less than 3 xc3x85 nor more than 30 xc3x85, and more preferably not less than 3 xc3x85 nor more than 10 xc3x85. Consequently, sufficient ohmic characteristics can be obtained while preventing the first and second ohmic electrodes from being stripped.
The semiconductor device may further comprise a fifth metal film which is formed in contact with the second metal film and is composed of gold, and a sixth metal film which is formed in contact with the fourth metal film and is composed of gold.
Thus, wire bonding can be easily performed. In this case, since the first ohmic electrode composed of the first metal film and the second metal film as well as the second ohmic electrode composed of the third metal film and the fourth metal film can be formed without being alloyed by heat treatment, the first metal film, the second metal film and the fifth metal film as well as the third metal film, the fourth metal film and sixth metal film can be formed in the same process.
A method of fabricating a semiconductor device according to still another aspect of the present invention comprises the steps of forming a p-type nitride based semiconductor; and forming a first metal film having a thickness of not less than 3 xc3x85 nor more than 150 xc3x85 which is composed of titanium and a second metal film which is composed of platinum or palladium in this order in contact with the p-type nitride based semiconductor, to form an ohmic electrode without carrying out heat treatment.
In the fabricating method, the first metal film composed of titanium has the function of cleaning the surface of the p-type nitride based semiconductor, to make ohmic contact between the second metal film composed of platinum or palladium and the p-type nitride based semiconductor easy. Consequently, the ohmic electrode is formed in a simple process without being alloyed by heat treatment.
Since the first metal film composed of titanium has a strong adhesive force to the p-type nitride based semiconductor, an adhesive force of the ohmic electrode to the p-type nitride based semiconductor is improved. Consequently, the ohmic electrode is not easily stripped.
Particularly, the thickness of the first metal film is preferably not less than 3 xc3x85 nor more than 130 xc3x85, more preferably not less than 3 xc3x85 nor more than 100 xc3x85, still more preferably not less than 3 xc3x85 nor more than 50 xc3x85, and most preferably not less than 3 xc3x85 nor more than 10 xc3x85. Consequently, sufficient ohmic characteristics can be obtained while preventing the ohmic electrode from being stripped.
A method of fabricating a semiconductor device according to still another aspect of the present invention comprises the steps of forming an n-type nitride based semiconductor; and forming a first metal film having a thickness of not less than 3 xc3x85 nor more than 100 xc3x85 which is composed of titanium and a second metal film which is composed of platinum or palladium in this order in contact with the n-type nitride based semiconductor, to form an ohmic electrode without carrying out heat treatment.
In the fabricating method, the first metal film composed of titanium has the function of cleaning the surface of the n-type nitride based semiconductor, to make ohmic contact between the second metal film composed of platinum or palladium and the n-type nitride based semiconductor easy. Consequently, the ohmic electrode is formed in a simple process without being alloyed by heat treatment.
Since the first metal film composed of titanium has a strong adhesive force to the n-type nitride based semiconductor, an adhesive force of the ohmic electrode to the n-type nitride based semiconductor is improved. Consequently, the ohmic electrode is not easily stripped.
Particularly, the thickness of the first metal film is preferably not less than 3 xc3x85 nor more than 50 xc3x85, more preferably not less than 3 xc3x85 nor more than 30 xc3x85, and still more preferably not less than 3 xc3x85 nor more than 10 xc3x85. Consequently, sufficient ohmic characteristics can be obtained while preventing the ohmic electrode from being stripped.
In a method of fabricating a semiconductor device according to still another aspect of the present invention comprises the steps of forming a p-type nitride based semiconductor and an n-type nitride based semiconductor which are brought into contact with each other; and forming a first metal film having a thickness of not less than 3 xc3x85 nor more than 100 xc3x85 which is composed of titanium and a second metal film which is composed of platinum or palladium in this order in contact with the p-type nitride based semiconductor, to form a first ohmic electrode without carrying out heat treatment, and forming a third metal film having a thickness of not less than 3 xc3x85 nor more than 100 xc3x85 which is composed of titanium and a fourth metal film which is composed of platinum or palladium in this order in contact with the n-type nitride based semiconductor, to form a second ohmic electrode without carrying out heat treatment.
In the fabricating method, the first and third metal films composed of titanium respectively have the functions of cleaning the surfaces of the p-type nitride based semiconductor and the n-type nitride based semiconductor, to make ohmic contact between the second metal film composed of platinum or palladium and the p-type nitride based semiconductor and ohmic contact between the fourth metal film composed of platinum or palladium and the n-type nitride based semiconductor easy. Consequently, the ohmic electrode is formed in a simple process without being alloyed by heat treatment.
Since the first and third metal films composed of titanium respectively have strong adhesive forces to the p-type nitride based semiconductor and the n-type nitride based semiconductor, respective adhesive forces of the first ohmic electrode and the second ohmic electrode to the p-type nitride based semiconductor and the n-type nitride based semiconductor are improved. Consequently, the first and second ohmic electrodes are not easily stripped.
Furthermore, the first ohmic electrode and the second ohmic electrode have the same structure, and are composed of the same material. Accordingly, the first and second ohmic electrodes can be simultaneously formed in the same process. Consequently, the number of fabricating processes is reduced, thereby making it possible to shorten fabrication time.
As a result, ohmic electrodes which are stable and reliable can be easily formed, respectively, on the p-type nitride based semiconductor and the n-type nitride based semiconductor.
The thickness of the first and third metal films is preferably not less than 3 xc3x85 nor more than 30 xc3x85, and more preferably not less than 3 xc3x85 nor more than 10 xc3x85. Consequently, sufficient ohmic characteristics can be obtained while preventing the first and second ohmic electrodes from being stripped.
The fabricating method may comprise the step of forming a fifth metal film composed of gold in contact with the second metal film and forming a sixth metal film composed of gold in contact with the fourth metal film.
Thus, wire bonding can be easily performed. In this case, since the first ohmic electrode composed of the first metal film and the second metal film as well as the second ohmic electrode composed of the third metal film and the fourth metal film can be formed without being alloyed by heat treatment, the first metal film, the second metal film and the fifth metal film as well as the third metal film, the fourth metal film and sixth metal film can be formed in the same process.
The foregoing and other object, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.